A tracer-based assessment of hydrological pathways at different spatial scales in a mesoscale watershed in NE Scotland

Geochemically based hydrograph separation techniques were used in a preliminary assessment to infer how runoff processes change with landscape characteristics and spatial scale (1-233 km(2)) within a mesoscale catchment in upland Scotland. A two-component end-member mixing analysis (EMMA) used Gran alkalinity as an assumed conservative tracer. Analysis indicated that, at all scales investigated, acidic overland flow and shallow subsurface storm flows from the peaty soils covering the catchment headwaters dominated storm runoff generation. The estimated groundwater contribution to annual runoff varied from 30% in the smallest (ca 1 km(2)) peat-dominated headwater catchment with limited groundwater storage, to >60% in larger catchments (>30 km(2)) with greater coverage of more freely draining soils and more extensive aquifers in alluvium and other drift. This simple approach offers a useful, integrated conceptualization of the hydrological functioning in a mesoscale catchment, which can be tested and further refined by focused modelling and process-based research. However, even as it stands, the simple conceptualization of system behaviour will have significant utility as a tool for communicating hydrological issues in a range of planning and management decisions. Copyright (C) 2002 John Wiley Sons, Ltd.